Venus could have been habitable while life evolved on Earth

But a mere one or two billion years ago, these two wayward siblings might have been more alike. New computer simulations suggest that early Venus might have looked a lot like our home planet – and it might even have been habitable.

“It’s one of the big mysteries about Venus. How did it get so different from Earth when it seems likely to have started so similarly?” says David Grinspoon at the Planetary Science Institute in Tucson, Arizona. “The question becomes richer when you consider astrobiology, the possibility that Venus and Earth were very similar during the time of the origin of life on Earth.”

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Grinspoon and his colleagues aren’t the first to imagine that Venus was once hospitable. It’s similar to Earth in size and density, and the fact that the two planets formed so close together suggests that they’re made of the same bulk materials. Venus also has an unusually high ratio of deuterium to hydrogen atoms, a sign that it once housed a substantial amount of water, mysteriously lost over time.

Venus, but snowy

To simulate early Venus, the researchers turned to a model of environmental conditions often used to study climate change here on Earth. They created four versions for Venus, each varying slightly in details such as the amount of energy the planet received from the sun, or the length of a Venusian day. Where information was scant about Venus’s climate, the team filled in educated guesses. They also added a shallow ocean, 10 per cent the volume of Earth’s ocean, covering about 60 per cent of the planet’s surface.

Looking at how each version might have evolved over time, the researchers say they were encouraged to believe that the planet might have looked much like an early Earth, and remained habitable for a substantial portion of its lifetime. The most promising of the four Venuses enjoyed moderate temperatures, thick cloud cover and even the occasional light snowfall.

Could life have emerged on this early Venus? If it did, it’s certainly no more, thanks to the oceans later boiling away and volcanoes drastically reshaping the landscape around 715 million years ago. But the team is not ruling it out.

“There’s great uncertainties in understanding Earth, not only its climate history but the history of how life began,” says Michael Way at the NASA Goddard Institute for Space Studies in New York City. If it began in oceans on Earth – a theory we’ve yet to confirm – the same could be true on a waterlogged Venus. “There’s no reason that life on this world would not have existed in these oceans. But that’s about all you can say.”

Alternative histories

“Both planets probably enjoyed warm liquid water oceans in contact with rock and with organic molecules undergoing chemical evolution in those oceans,” says Grinspoon. “As far as we understand at present, those are the requirements for the origin of life.”

To bolster their findings, the team suggests a future mission to Venus should look out for signs of water-related erosion near the equator, which would provide evidence for the oceans detailed in their simulation. Such signs have already been detected by missions at Mars. NASA is currently weighing up two potential Venus projects, although neither has been confirmed. One mission would drop a probe through the clouds down to the surface, while another would orbit around the planet and image its surface.

The researchers would also like to run simulations of further alternative pasts for Venus – perhaps one where it was a desert world, or submerged in as much water as Earth, to find out which scenario is most likely to lead to the Venus we see today.

The study could also aid astronomers in their search for exoplanets, says James Kasting at Pennsylvania State University. If Venus might have once been habitable, then it suggests that other planets close to their stars might be, too. “If you make the habitable zone really wide, that raises the probability of finding an Earth.”